This invention relates to methods by which the natural, genetically determined fecundity of certain livestock may be manipulated and altered. Primarily it is concerned with methods whereby individual animal fecundity of sheep or goats in a flock may be increased to a degree that significantly exceeds natural levels. It is further concerned with substances that can bring about the biological effects of the invention and particularly the manner in which they may be used.
Because of their considerable economic importance, studies of the reproductive biology of domestic livestock have long been directed toward processes by which the fecundity of farm animals could be manipulated. In the animal production industry it is well known that there occurs from time to time within a species of sheep, for example, a strain that exhibits greater natural fecundity than others of that species. Frequently the basis for this is a genetically heritable phenomenon which, by poorly understood mechanisms, produces significantly increased multiple ovulations in animals of the more fecund strain and which is at the centre of all attempts to increase natural fecundity by animal breeding programs.
Clearly for the natural fecundity of livestock to be increased there must be at fertilization a pattern of increased ovulation rate. Though demonstration of an increased ovulation rate is a necessary requirement for increased fecundity it is by no means a sufficient determinant because of the incidence of embryo losses that may occur in a particular species throughout pregnancy.
Nevertheless, it is already known in the prior art that by artificially increasing the ovulation rate in a domestic livestock species by the administration of pregnant mare serum gonadotrophin for example, it is possible to increase the ovulation rate and ultimately the overall fecundity of the species.
In a separate prior art it was also shown that by immunization of sheep to produce antibodies directed against certain endogenous hormones, the ovulation rate of those animals could be increased. The hormones were steroids of the oestrogen and androgen classes.